The present invention relates generally to gas turbines having closed cooling circuits in one or more nozzle stages and particularly relates to reducing thermally induced stresses in the inner and outer bands of the nozzle stages caused by temperature differentials between the hot gases of combustion flowing along the hot gas path and the cooling medium.
In industrial or land-based gas turbines, one or more of the nozzle stages are cooled by passing a cooling medium from a plenum in each nozzle segment portion forming part of the outer band through one or more nozzle vanes to cool the nozzles and into a plenum in the corresponding inner band portion. The cooling medium then flows radially outwardly from the inner band portion, again through the one or more nozzle vanes for discharge. Typically, the cooling medium is steam. Each of the nozzle segments including the inner and outer band portions and one or more nozzle vanes are typically cast. Covers are applied to the inner and outer band portions on sides thereof remote from the hot gas path to define plenums for receiving the cooling medium. The covers are not cast with the nozzle segments. Rather, they are preferably later applied to the inner and outer band portions, for example, by welding or brazing. With this arrangement, the hot gas flowpath sides of the bands are exposed to relatively high temperatures, while the covers which are not directly exposed to the hot gases of combustion along the flowpath, remain considerably cooler. Additionally, the covers are exposed externally to compressor discharge air which, while having a temperature higher than the temperature of the steam cooling medium is still considerably less than the temperature of the inner and outer bands exposed to the hot gases of combustion. The temperature differential between the covers and the band portions, particularly along the weld lines between the covers and walls of the band portions exposed to the hot gas path cover results in high thermal stresses. As a consequence, there is a need to reduce the thermally induced stresses along the inner and outer bands of the nozzle stages caused principally by temperature differentials between the hot gases of combustion in the hot gas path, the cooling medium flowing through the inner and outer bands and the compressor discharge air.
In accordance with a preferred embodiment of the present invention, the temperature difference between the flowpath exposed surfaces of the inner and outer bands and the covers exposed both to the cooling medium and the compressor discharge air is reduced by flowing a thermal medium along the covers at a temperature intermediate the temperature of the hot gases of combustion and the cooling medium through the cover and particularly adjacent the joints between the covers and the nozzle bands. The thermal medium flowing along the covers is at a significantly higher temperature than the temperatures of the cooling medium and the compressor discharge air in order to heat the cover so that the cover temperature approaches the bulk temperature of the flowpath exposed surfaces of the nozzle bands. To provide such thermal medium, a portion of the combustion path gases are directed through entry ports at the leading edges of the cover. Those gases follow passages through the cover and distribute heat substantially evenly to the cover for exit at the trailing edges of the covers into the hot gas path. Because of their very high temperature, flowpath gases alone can cause damage to the cover by way of oxidation, elevation of the bulk temperature of the covers in excess of that of the flowpath surfaces, and a reverse temperature gradient, resulting in similar high thermal stresses. To optimize the temperature of the thermal medium flowing through the heating passages in the covers, hot gases of combustion are combined with high pressure compressor discharge air for flow through the one or more passages in the cover. By providing one or more metering apertures in communication with compressor discharge air and with the passage(s) through the covers, hot flowpath gases entering the passage(s) are combined with compressor discharge air. This results in a thermal medium having a temperature sufficiently high to heat the cover adequately to reduce thermal stresses while avoiding the aforementioned and other problems.
Also, and advantageously, the mixture of hot combustion gases and compressor discharge air is (i) lower in pressure than both the compressor discharge air and hot gases of combustion at the leading edge of the passages and (ii) higher than the pressure of the hot gases of combustion at the trailing edge of the cover. Thus, the cooling medium flows passively through the passages between the leading edges to the trailing edges of the nozzle segments. The result is a cover having a temperature very close to the bulk temperature of the hot gas flowpath surfaces, thus reducing the thermal stresses induced by the thermal mismatch and affording higher component life and more reliable joints.
In a preferred embodiment according to the present invention, there is provided apparatus for controlling a temperature mismatch in at least one of the inner and outer bands of turbine nozzles having cooling circuits for flowing a cooling medium, comprising a nozzle segment having at least one nozzle vane and inner and outer nozzle band portions adjacent opposite ends of the nozzle vane and in part defining a path for flowing hot gases of combustion, one of the band portions forming a wall exposed to the hot gas path of the turbine and having a cover on a side of the wall remote from the hot gas path, the cover and the wall defining a plenum therebetween for receiving the cooling medium forming part of the cooling circuit, the segment including at least one passage through the cover for flowing a thermal medium at a temperature intermediate the temperature of the cooling medium and the hot gases of combustion to reduce the temperature differential between the cover and the wall and thereby reduce thermal-induced stresses in the one band portion.
In a further preferred embodiment according to the present invention, there is provided apparatus for controlling a temperature mismatch in at least one of inner and outer bands having a turbine nozzle vane therebetween and a cooling circuit for flowing a cooling medium through the nozzle vane, comprising a nozzle segment having at least one nozzle vane and inner and outer nozzle band portions adjacent opposite ends of the nozzle vane and in part defining a path for flowing hot gases of combustion, one of the band portions forming a wall exposed to a hot gas path of the turbine and having a cover on a side of the wall remote from the hot gas path, the cover and the wall, defining a plenum therebetween for receiving the cooling medium forming part of a nozzle cooling circuit, the cover and the wall of the band forming joints therebetween and along opposite sides thereof, the segment including passages through the cover from adjacent a leading edge to a trailing edge thereof and adjacent the joints for flowing the medium at a temperature intermediate the temperature of the cooling medium and the hot gases of combustion to reduce the temperature differential between the cover and the wall in the region of the joints to reduce thermal induced stresses in the one portion.
In a still further preferred embodiment according to the present invention, there is provided a method of reducing a temperature differential between a wall of an inner or an outer band of a turbine nozzle segment having a vane between the walls and a cover on a side of the wall remote from a flowpath for hot gases of combustion past the nozzle wherein the wall and cover define a plenum therebetween for receiving a cooling medium for flow through the nozzle vane, comprising the steps of flowing a thermal medium through at least one passage in the cover at a temperature intermediate respective temperatures of the hot gases of combustion and the cooling medium to elevate the temperature of the cover.